An apparatus described in JP-10-220272A (U.S. Pat. No. 6,142,121) is proposed as an apparatus of this kind, In a common rail type fuel injection system constructed of this fuel injection apparatus, fuel pressure-fed from a fuel pump is accumulated in a high-pressure state by a common rail. Then, the accumulated high-pressure fuel is supplied to the fuel injection valve of each cylinder through pipes (high-pressure fuel passage) disposed for each cylinder. The common rail is provided with a specified pressure sensor (rail pressure sensor). This system is constructed in such a way as to control the driving of various devices constructing a fuel supply system on the basis of the output of the sensor from this rail pressure sensor.
In recent years, the need for improving exhaust emission is increasing more and more in a diesel engine for an automobile. A study has been conducted in which injection characteristics including the timing of starting/finishing fuel injection and an injection rate are estimated to control the driving of the fuel injector by use of the estimation result. In this case, the pressure in the common rail (rail pressure) is measured by a rail pressure sensor disposed in the common rail and the injection characteristics are estimated on the basis of a change in the rail pressure.
However, according to the findings of the inventors of this application, when fuel is injected, a variation is developed in the rail pressure but the variation in the rail pressure is very little, so that it is thought that the variation in the rail pressure cannot be effectively used for estimating the injection characteristics with high accuracy. In other words, a pressure variation caused by an injection action is damped before the pressure variation reaches the common rail from the fuel injection port (injection opening) of the fuel injector and hence does not cause a variation in the rail pressure. Hence, there is a room for improvement as a technology for estimating injection characteristics.